1. Field of the Invention
The present invention relates to a motor and an electronic apparatus using the same.
2. Description of the Related Art
In electronic apparatuses such as laser printers, a paper feed roller (driven member) provided in a main body case is coupled via a deceleration mechanism to a driving shaft of a motor. When this motor is driven, the paper feed roller rotates and feeds paper to a predetermined portion.
As this motor, a brushless DC motor that ordinarily is used includes: a stator on whose outer circumference a plurality of magnetic poles are arranged at a first predetermined interval; and a rotor that is rotatably disposed around the stator; wherein an inner circumferential face of the rotor is provided with a magnet magnetized to have opposite polarities at a second predetermined interval (main magnetization).
In this sort of motor, ordinarily in order to arrange the magnet of the rotor as close as possible to a magnetism-detecting element that magnetically detects rotation of the rotor, the size of the magnet in a direction parallel to a motor-driving shaft is set larger than the size of a magnetic pole base of the stator in the same direction. In this case, an extended portion, called an “end plate”, that extends in a direction substantially parallel to the magnet often is formed on both sides of a magnetic pole base, at outer circumferential ends of the magnetic poles of the stator (see JP H9-285044A and JP 2007-244004A, for example). Accordingly, the area in which the magnet of the rotor and the magnetic poles of the stator oppose each other increases, and, thus, the driving force and the driving efficiency of the motor can be increased.
Furthermore, for example, in the case where a paper feed roller of a laser printer is driven via a deceleration mechanism, the rotation of a brushless DC motor has to be controlled precisely. Accordingly the rotational speed of the brushless DC motor has to be detected at a certain level of resolution.
As a speed-detecting method appropriate for this sort of purpose, a FG method (described later) ordinarily is used. That is to say, the magnet of the rotor is magnetized to generate a torque (main magnetization), and, moreover, multi-pole magnetization (FG magnetization) in a direction opposing the substrate is performed on a face of the magnet opposing a substrate. Furthermore, the substrate is provided with a FG pattern in the circumferential direction, in which linear elements in the same number as that of magnetized poles of the FG magnetization are connected in series. When the rotor rotates, an induced voltage is generated at the linear elements due to magnetic fluxes obtained by the FG magnetization, and a speed detection signal (FG signal) at a frequency proportional to the rotations of the motor can be obtained through this FG pattern.
In this sort of FG method, in order to reduce the influence of magnetic fluxes obtained by the main magnetization, there is a known method for canceling the influence of magnetic fluxes obtained by the main magnetization, by configuring the FG pattern from a main pattern and a cancellation pattern and connecting the main pattern and the cancellation pattern in series (see JP 2006-25537A, for example).
However, in the case where an extended portion that extends in a direction substantially parallel to the magnet is provided at outer circumferential ends of the magnetic poles of the stator, it may be difficult to cancel the influence of magnetic fluxes obtained by the main magnetization in the above-described FG method. The reason is as follows.
The extended portion provided at the outer circumferential ends of the magnetic poles has a magnetism collecting effect, and, thus, most of magnetic fluxes obtained by the main magnetization are drawn into the extended portion. However, part of the magnetic fluxes flowing into the extended portion leaks out of the extended portion due to magnetic saturation of the extended portion, and forms leakage magnetic fluxes. These leakage magnetic fluxes significantly affect a portion of the FG pattern close to the stator (i.e., a portion on the inner circumferential side of the FG pattern in the radial direction). Accordingly, the influence of the leakage magnetic fluxes obtained by the main magnetization differs between a portion of the FG pattern close to the stator and a portion away from the stator in the radial direction, and the influence of the leakage magnetic fluxes obtained by the main magnetization cannot be canceled sufficiently. As a result, noise is superimposed on the FG signal, and the precision in detecting the rotational speed is lowered.